This invention relates to a control system for a gas turbine having a fuel gas boost compressor to boost the fuel gas pressure before supplying the fuel gas to the turbine, and more particularly to a control system to control the combination of a gas boost compressor and gas turbine in such a manner as to meet the load demand with a minimum of power required to drive the fuel gas boost compressor.
The development and improvement of the gas turbine over the years has led to improved turbine performance utilizing increased mass flow, firing temperature and pressure ratio. This has led to the need for increased fuel gas supply pressure, and present day requirements are typically in the range of 275-325 PSIG (pounds per square inch), with higher pressures to as much as 400 to 600 PSIG required for current aero derivative engines and new heavy duty systems presently under development. While the fuel gas supply pressure requirements have been increasing the availability of fuel gas supplies at appropriate pressures has been decreasing. This in part due to the use of fuel gas other than natural gas, such as process gasses, which may be available at many industrial and cogeneration sites where gas turbines are used in the generation of electric power. These fuels are often available at very low, or in some instances, atmospheric pressure. This has led to the use of gas boost compressors which take the available fuel gas and boost its pressure to that required prior to supplying it to the gas turbine.
The required use of fuel gas boost compressors has added considerable cost to operation in addition, of course, to the initial cost of the equipment which may amount to 10 or 12 dollars per kilowatt of system power output for simple cycle units, and 20-25 dollars per kilowatt for dual redundant gas boost compressor installations. However, a second significant cost element for the system is the cost of operation of the gas boost compressors, that is the cost of the power necessary to drive the fuel gas boost compressor. This absorbed power reduces the net power plant output. The present invention cannot eliminate the need for a fuel gas boost compressor but it can significantly reduce the fuel gas boost compressor power consumption. An example of the fuel cost for a gas boost compressor may be calculated as follows: assuming four dollars per million BTU (British Thermal Units) and 8,000 hours of operation per year of the power plant with a normal gas boost compressor power requirement of 1200 Kilowatts, the annual equivalent fuel cost for the compressor power is $345,000. Even though the gas boost compressor fuel consumption is only 2-3 percent of the fuel gas requirement, an improved control system which could save up to 40 percent of the boost compressor fuel would result in considerable annual savings. Over the life of the generating plant, this can amount to millions of dollars.